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By Duncan Graham-Rowe WE WALK about with thousands of songs, photos and videos in our pockets, but we won’t be able to keep cramming more and more onto our memory cards forever. While last week’s news that a single bit of digital information has been stored on just 12 atoms was remarkable, at some point we are going to reach a limit. What happens when we can store a single bit on an atom, for example? Now a radical approach could solve our impending memory crisis and allow a massive increase in storage density – without making chips any bigger. It may also force us to reassess the notion that all things digital must be made up of 1s and 0s. The idea is simple: why use a single memory cell to store two binary states when it could hold many more? The technology relies upon phase change materials (PCMs) that can hold information by switching between an amorphous state and a crystalline one. PCM memory can write and retrieve data 100 times faster than Flash memory, which is used in many consumer gadgets and computers. It is also extremely durable and can be reused at least 10 million times; Flash can cope with just 3000 uses. Why use a single memory cell to store two binary states when it could hold many more? But PCM memory’s true potential lies in its ability to store more than a single bit per cell. “If you are able to control the current you can create states between the two, something that is not fully crystallised and something that is not fully amorphous,” says Evangelos Eleftheriou, head of storage technologies at IBM’s Zurich Research Laboratory in Switzerland. Precisely how many states can be created remains to be seen, but some researchers, like David Wright at the University of Exeter in the UK, have already demonstrated 512 discrete states in a single 20-nanometre cell – about the same size as a Flash memory cell, which usually only holds two. Challenges still remain, however. Differentiating between these distinct states requires highly sensitive and expensive equipment, which wouldn’t be practical in a chip, says Wright. Another issue is drift, where the resistance of the material changes over time. This is not a problem when storing two states but could be a nuisance for multiple states. IBM believes it has a two-fold solution: electrically measuring the amorphous thickness of the material instead of the resistance, and reading multiple cells at the same time to gauge their relative drifted positions. For now most research is focused on the slightly less ambitious goal of storing four states, or two bits of data per cell, says Eleftheriou. “It gives you double the capacity for more or less the same cost,” he says. So is this the death of binary? Not necessarily. Storing multiple bits doesn’t mean we have to abandon binary – and the decades of technology and programming that come with it – and switch to base-4. Four states can be just as easily used to store two binary bits as it can four separate pieces of data: in information terms they are the same. However, the PCM revolution is not here yet, says Wright. The three big memory companies – Samsung, Micron and Hynix – are unlikely to start rolling out multistate PCM until they have finished squeezing revenues from Flash, says Eleftheriou, which will probably be in 2016. “They will really milk that cow as much as they can,